1. Field of the Invention
The present invention relates generally to nonaqueous electrolyte elements for use in secondary electric current-producing cells, and to rechargeable lithium electric current-producing cells employing a nonaqueous electrolyte element comprising a soluble electrolyte additive that increases the lithium stripping efficiency at the anode-electrolyte interface.
2. Prior Art
As the rapid evolution of batteries continues, and particularly as lithium-ion and lithium metal batteries become more widely accepted for a variety of uses, the need for safe, long lasting rechargeable batteries becomes increasingly important. There has been considerable interest in recent years in developing high energy secondary batteries comprising an electrolyte element which improves the cycle life and safety of the battery. U.S. Pat. Nos. 5,460,905 and 5,462,566 by applicant, the disclosures of both of which are totally incorporated herein by reference, describe the basic elements and performance requirements of secondary lithium batteries and their components.
The production of unitary electrolyte elements which are particularly useful in electrochemical cells of many different types in general, have been extensively described in patents and other literature. Some of these electrolyte elements have multilayer structures and are prepared by various means of adding an additional electrolyte layer between a first electrolyte layer and one or both of the electrodes. Alternatively, if desired, a barrier or stabilization layer may be imposed between the electrolyte element and one or both of the electrodes, such as described in U.S. Pat. No. 5,487,959 to Koksbang.
One of the many problems encountered in the process of producing electrolyte elements, and particularly those useful in electrochemical cells with lithium as the negative electrode or anode, is that there is a difficulty in obtaining good efficiency, cycling life, and safety of the cells due to reactivity of the electrolyte element with the electrode elements, particularly due to reactions with the anode. There is further difficulty in obtaining good efficiency, cycling life, and safety of the cells due to the diffusion of materials from other layers of the cell, particularly discharge products from the positive electrode or cathode layer, into the electrolyte element. These materials that have diffused into the electrolyte element can react in a detrimental way with the anode or another layer in contact with the electrolyte element, or can react directly with the electrolyte element.
The foregoing disadvantages for producing electrolyte elements for electrochemical cells are even more problematic when the cathode layer utilizes a sulfur-based active material in combination with a lithium-based anode. For instance, in U.S. Pat. No. 3,907,591, Lauck describes the reduction of the elemental sulfur active material during the discharging of the cell to produce soluble lithium polysulfides at high concentrations. This leads to reduced efficiency and cycling life of the cells due to formation of insoluble lithium sulfides, such as lithium sulfide (Li.sub.2 S), which deposit on the cathode and clog its pores to block interaction with the electrolyte, as well as due to undesirable reactions of the soluble lithium polysulfides with the lithium-based anode.
Lithium and sulfur are highly desirable as the active materials for the anode and cathode, respectively, of rechargeable or secondary battery cells because they have the highest energy density on a weight or volume basis of any of the known combinations of active materials. To obtain high energy densities, the lithium can be present as the pure metal or in an alloy or in an intercalated form, and the sulfur can be present as elemental sulfur or as an organic material with a high sulfur content, preferably greater than 50 weight per cent sulfur. The aforementioned disadvantages of the high reactivity of lithium with the electrolyte element and the transport of excessive amounts of soluble and insoluble polysulfides from the cathode layer into the electrolyte element in combination cause a complex number of chemical reactions which in total reduce the cycle life and safety of the cells.
To overcome the foregoing disadvantages for producing electrolyte elements, and in particular for producing electrochemical cell elements with a lithium-based anode, in U.S. Pat. No. 4,303,748 to Armand, et al., the disclosures of which are totally incorporated herein by reference, the electrolyte element is a solid polymer electrolyte (SPE). This SPE is less reactive with the lithium-based anode than liquid electrolytes and, optionally, an ionically conductive polymer, the same as or similar to that in the SPE, is further incorporated into the cathode composite layer containing, for example, elemental sulfur and conductive carbon. The ionically conductive polymer is incorporated into this elemental sulfur and conductive carbon coating at levels of up to 25 weight per cent to improve the electrochemical performance and the mechanical integrity of the coating. In spite of the lower reactivity of the electrolyte element and the binding strength and ionic conductivity of the polymer, such as polyethylene oxide (PEO), in the composite cathode, there remains excessive diffusion of soluble lithium polysulfides from the cathode into the electrolyte element and into contact with the lithium anode which severely reduces the cycle life and safety of the cell.
In U.S. Pat. No. 5,523,179, the disclosures of which are totally incorporated herein by reference, Chu describes some of the prior art on elemental sulfur cathode/lithium anode battery cells, including the formation and detrimental action of polysulfides with nonaqueous electrolytes. No information is provided on the use of additives in the electrolyte element to increase cycle life and safety.
U.S. Pat. Nos. 4,833,048 and 4,917,974 to De Jonghe, et al., disclose a class of sulfur-based cathode materials made of organosulfur compounds of the formula (R(S).sub.y).sub.n where y=1 to 6; n=2 to 20, and R is one or more different aliphatic or aromatic organic moieties having one to twenty carbon atoms. The preferred form of the cathode material is a simple dimer or (RS).sub.2. Herein, by the term "organosulfur composition" is meant a composition containing organic sulfur compounds with only single or double carbon-sulfur bonds or sulfur-sulfur bonds forming disulfide linkages, and typically with more than 3 per cent by weight of non-sulfur or non-carbon elements. The organosulfur materials investigated by De Jonghe, et al., undergo polymerization (dimerization) and de-polymerization (disulfide cleavage) upon the formation and breaking of the disulfide bonds. The de-polymerization which occurs during the discharging of the cell results in lower molecular weight polymeric and monomeric species, namely soluble anionic organic sulfides, which can dissolve into the electrolyte and cause self-discharge as well as reduced capacity, thereby severely reducing the utility of the organosulfur material as a cathode-active material and eventually leading to complete cell failure. The result is an unsatisfactory cycle life having a maximum of about 200 deep discharge-charge cycles, more typically less than 100 cycles as described in J. Electrochem. Soc., Vol. 138, pp. 1891-1895 (1991). Although the soluble discharge products are soluble sulfides rather than the polysulfides of the type formed with elemental sulfur, the detrimental effects on efficiency and cycle life are similar. In addition, the organosulfur materials typically contain less than 50 weight per cent of sulfur so they have correspondingly a much lower energy density than elemental sulfur.
U.S. Pat. No. 5,441,831 and U.S. patent application Ser. No. 08/478,330 now U.S. Pat. No. 5,601,947 by applicant, the disclosures of which are totally incorporated herein by reference, disclose carbon-sulfur polymers of the general formula I EQU --(CS.sub.x).sub.n -- I
wherein x takes values of 1.2 and greater and n is an integer equal to or greater than 2. Herein, by the term "carbon-sulfur polymer composition" is meant a composition containing carbon-sulfur polymers with carbon-sulfur single and double bonds, with sulfur-sulfur bonds forming disulfide, trisulfide, and higher polysulfide linkages, and typically with less than 3 per cent by weight of non-sulfur or non-carbon elements. Further useful carbon-sulfur cathode active polymers are compositions of general formula II, EQU --(C.sub.2 S.sub.z).sub.n -- II
wherein z ranges from greater than 1 to about 100, and n is equal to or greater than 2, as described in U.S. patent application Ser. Nos. 08/477,106 now U.S. Pat. No. 5,529,860 and 08/602,323 by applicant, the disclosures of which are totally incorporated herein by reference. With these carbon-sulfur cathode active compositions, organic polysulfides are formed during discharge. Polysulfides are meant to indicate sulfides with two or more sulfur atoms bonded together. Thus, the disulfides of the organosulfur compositions described heretofore form monosulfides or sulfides RS.sup.- ! during reduction or discharge. Since the carbon-sulfur polymer compositions of I and II contain large amounts of --S.sub.m --! groups where m is 3 or greater, they form organic polysulfides R'S.sub.x.sup.- !, where x is 2 or greater and R' is the carbon-sulfur moiety to which the polysulfide group is attached, during reduction or discharge. Some of these organic polysulfides are insoluble because of their attachment to the polymer backbone, but upon continued discharge, they progressively are further reduced to soluble organic polysulfides, and still further to soluble inorganic polysulfides S.sub.x.sup.2- !, where x is 2 or greater. Thus, with the carbon-sulfur polymer compositions, some of the discharge products are the same polysulfides formed in the discharge of cathodes containing elemental sulfur.
Even though the carbon-sulfur polymer compositions show improvements over organosulfur compositions as cathode active materials because of lower amounts of soluble sulfides and because of a higher energy density from the typically higher weight per cent of sulfur of over 50 per cent, and preferably above 85 per cent, there is still some formation of soluble organic polysulfides, as well as inorganic polysulfides as also formed in the discharge of elemental sulfur cathode active compositions.
Several approaches have been described to inhibit or retard the transport or diffusion of soluble polysulfides from the cathode to the electrolyte element. U.S. Pat. No. 3,806,369 to Dey, et al., describes an ion exchange membrane between the cathode and the electrolyte/separator layer to inhibit the passage of polysulfides or other anions from the cathode into the electrolyte element. Without this barrier layer, the soluble polysulfides or other anions form insoluble films on the cathode and shorten the cycle life of the cell. U.S. Pat. No. 3,532,543 to Nole, et al., describes the largely unsuccessful attempt to use copper halide salts to limit the formation of polysulfides in an elemental sulfur cathode.
In a provisional U.S. patent application, filed on May 22, 1996, by applicant, the disclosures of which are totally incorporated herein by reference, there is disclosed the addition of a class of materials to the sulfur-based cathode active material to encapsulate or entrap the sulfur-based material to effectively retard the transport of soluble polysulfides and sulfides from the cathode into the electrolyte element.
Barrier layers such as those described heretofore can be effective in preventing excessive diffusion of soluble cathode reduction products, such as inorganic polysulfides, into the electrolyte element, thereby improving cycle life and safety from the levels obtained when excessive inorganic polysulfides and other soluble cathode reduction products are present in the electrolyte element. However, these barrier layers have disadvantages, besides the cost and the non-cathode active volume occupied by the materials, in that they may so effectively block the transport of soluble anionic species into the electrolyte element that low, but not excessive, concentrations of the soluble anions that would have some beneficial effects in the electrolyte element are not obtained. Also, the barrier may be only partially effective so that there is a slow buildup of soluble cathode reduction products in the electrolyte, at first at too low concentrations to be beneficial in the early cycles of the cell which can be a particularly critical time period for the ultimate cycle life and safety of the cell. In the later charge-discharge cycles of the cell, the concentrations of the soluble polysulfide and other anions can become too high or excessive, thereby shortening the cycle life and decreasing the safety. Lastly, these barriers are typically indiscriminate in that they do not selectively allow passage of one soluble anion which is acceptable or desired in the electrolyte element while blocking the passage of another soluble anion which is not acceptable or desired in the electrolyte element.
There have been several types of battery cells which incorporate polysulfides in the electrolyte or the cathode. One type is the sodium-sulfur battery cell, such as described in U.S. Pat. No. 3,993,503 to Ludwig, the disclosures of which are totally incorporated herein by reference, which involves liquid sodium anodes and liquid cathodes with a solid electrolyte and with operation at elevated temperatures, well above room temperature, where the anode and cathode materials are in a molten state. Another type is the aqueous lithium-sulfur cell where the polysulfide is in an aqueous electrolyte, as described in U.S. Pat. No. 5,413,881 to Licht, et al., the disclosures of which are totally incorporated herein by reference. Both these types have electrochemical properties, operating conditions, and materials markedly distinct from the solid lithium metal/non-aqueous electrolyte/non-liquid cathodes of the secondary cells of the present invention.
There has been some mention of the beneficial effects of inorganic polysulfides in primary cells of the general solid lithium/non-aqueous electrolyte/elemental sulfur non-liquid cathode type. U.S. Pat. No. 4,410,609 to E. Peled, et al., the disclosures of which are totally incorporated herein by reference, describes the use of greater than 0.01M polysulfide in the electrolyte of a primary lithium-elemental sulfur cell in combination with greater than a 0.1 M concentration of a lithium salt to form an insoluble Li.sub.2 S solid electrolyte interphase or film on the anode material. Since it is directed to a primary cell only, no information is provided on electrolytes for use in rechargeable secondary lithium cells where additional requirements, such as improving the cycle life and safety, are particularly important.
More research on the addition of polysulfides to form a passivation film on lithium anodes is described in J. Electrochem. Soc., Vol. 135, pp. 1045 to 1048 (1988) by Yamin, et al., and in J. Electrochem. Soc., Vol. 136, pp. 1621 to 1625 (1989) by Peled, et al., and references therein, the disclosures of all of which are totally incorporated herein by reference.
For secondary cells of the solid lithium/non-aqueous electrolyte/non-liquid cathode type, E. Peled, et al., in J. Power Sources, Vol. 26, pp. 269 to 271 (1989), the disclosures of which are totally incorporated herein by reference, describes the presence of lithium polysulfide at a 0.1 M concentration in an electrolyte, but reported extremely low capacities and cycle life.
Besenhard, et al., in J. Power Sources, Vol. 43-44, pp. 413 to 420 (1993), the disclosures of which are totally incorporated herein by reference, describes a twofold decrease in cycle capacity loss for just the first two cycles with a lithium intercalated carbon anode and various inorganic additives, including a very low 0.0003M polysulfide concentration. They attribute this to the formation of a protective film on the anode surface which forms adequately after these two cycles even in the absence of additives in the electrolyte. The additives are described as beneficial for the filming process on the anode but the benefit is limited to just the first two cycles of the lithium intercalated carbon anode and electrolyte. The lithium salt used in this work, lithium perchlorate, is one of the poorer choices for secondary lithium cells with good cycle life and safety due to its reactivity and instability. Besenhard, et al., conclude on page 419 in this article that "in the case of metallic lithium electrodes however, the considerable electrochemical reactivity of all of these additives may be a serious drawback."
Other soluble additives to the electrolyte element of solid lithium anode/non-aqueous electrolyte/cathode type secondary cells besides polysulfides have been described. These include carbon dioxide and other inorganic additives in the aforementioned article by Besenhard, et al., including references therein; acid anhydrides as described in U.S. Pat. No. 5,296,319 to Bito, et al., to reduce or eliminate the presence of water; an unidentified reaction product of carbon disulfide and lithium, possibly a soluble sulfide, in the aforementioned U.S. Pat. No. 3,532,543; and high concentrations of water in U.S. Pat. Nos. 5,432,425 and 5,436,549 to Lundquist, et al. These additives often have disadvantages in that they are only effective for the first few cycles of cell use or they are not effective when the cathode material is sulfur-based. Also, they often are not compatible with the preferred solvents, lithium salts, and other materials of the non-aqueous electrolyte element of a lithium secondary cell.
It would therefore be advantageous to be able to utilize a material useful in the non-aqueous electrolyte element of a lithium secondary cell which exhibits beneficial effects on cycle life and safety during the first cycles of the charge-discharge cycles of the cell and maintains its beneficial effects during the useful life of the cell and which can be incorporated easily and reliably into the cell without significant extra cost.
It is therefore an object of the present invention to provide a soluble additive to the non-aqueous electrolyte which is suitable for use in manufacturing secondary lithium cells and which can be conveniently added to the electrolyte at the same time that one or more other additives to the electrolyte are added.
Another object of the present invention is to provide a means to identify such a soluble additive and to determine the range of amounts of its beneficial use in the non-aqueous electrolyte.
It is another object of the present invention to provide such a soluble electrolyte additive and non-aqueous electrolyte that is useful with both lithium metal and lithium ion anodes for secondary battery cells.
Yet another object of the present invention is to provide such a soluble electrolyte additive and non-aqueous electrolyte which is suitable to increase the cycle life and safety of secondary lithium cells.
It is another object of the present invention to provide such a soluble electrolyte additive and non-aqueous electrolyte which is present and useful in the initial discharge-charge cycles of the secondary lithium cells.
Still another object of the present invention is to provide secondary lithium cells which maintain the amounts of such a soluble electrolyte additive within the desired range in the non-aqueous electrolyte during the useful cycle life of the cells and which prevent excessive amounts of cathode reduction discharge products from diffusing into the electrolyte.
Another object of the present invention is to provide such a soluble electrolyte additive that is compatible with all the other materials of the secondary lithium cell that are in contact with the additive and which helps to control the reactivity of the other materials of the electrolyte with the lithium anode and with the other materials of the cell that are in contact with the electrolyte.
Yet another object of the present invention is to provide such a soluble electrolyte additive and non-aqueous electrolyte which is useful with secondary lithium cells which utilize elemental sulfur, organosulfur, or carbon-sulfur polymer compositions as a cathode active material.
Still another object of the present invention is to provide a process for preparing a non-aqueous electrolyte suitable for use in producing lithium metal and lithium ion secondary cells, which have increased cycle life and safety.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.